1. Field of the Invention
The present invention relates to a switching circuit for a millimeter waveband control circuit, and more particularly, to a switching circuit for a millimeter waveband control circuit capable of securing its high isolation and low voltage loss using a switching cell.
The present invention was supported by the IT R&D program of MIC/IITA[2006-S-039-03, SoP (System on Package) for 60 GHz Pico cell Communication].
2. Description of the Related Art
A millimeter waveband of several tens GHz has been widely used in microwave communication systems such as wireless LAN, radar system for car accidents prevention system, image radar system, etc., and a switching circuit for converting these radio frequency signals has been mainly used as a converter circuit and a transmission/reception converter circuit in an antenna.
Field effect transistors (FETs) such as a high electron mobility transistor (HEMT) or a metal-semiconductor field effect transistor (MESFET) has been widely used as a core element in the switching circuit. Here, the field effect transistor (FET) is a compound semiconductor transistor that has good transmission and drive voltage characteristics in a millimeter waveband and low power consumption, includes a simple bias circuit, and facilitate the implementation and integration of its multiple ports.
For such a switching circuit, there is required a technology of lowering its insertion loss as low as possible and diminishing its isolation damage by parasite components such as inductance or capacitance, etc. In particular, it is very important to design a high-isolation switching circuit for the purpose of production of a small microwave control circuit.
For this purpose, a single-pole double-throw (SPDT) switching circuit, which is mainly used for the change in a transmitting/receiving path of an ultrahigh frequency signal, mainly employs only a shunt structure since its series-shunt structure has very high insertion loss and makes it difficult to ensure the isolation in a millimeter waveband of several tens GHz, and it is difficult to realize an isolation of −30 dB or less in the transmitting/receiving path, the an isolation of −30 dB or less being required for a transceiver.
The shunt structure functions to block a flow into an output port by coupling a ground via hole to a drain electrode or a source electrode of the switch element and forcing an undesirable signal to flow into the ground under the control of the a gate voltage as a control voltage according to the millimeter wave signal inputted into the drain electrode or the source electrode.
A multi-stage shunt technology has been widely used in the conventional shunt structure to ensure the high isolation. However, the use of the multi-stage shunt technology results in the increases in the chip size and the manufacturing cost due to the presence of inductors or capacitors that are additionally mounted in the surroundings such as a transmission line, a plurality of field effect transistors and a switch element in a λ/4 transformer
In order to solve the above problems, U.S. Pat. No. 6,320,476 (registered on Nov. 20, 2001) discloses a “millimeter-band semiconductor switching circuit” whose isolation is improved by minimizing a distance between a via hole and a transmission line.
However, the conventional switching circuits has a configuration in which a transmission line and a via hole are disposed vertically to each other so as to minimize a distance between the via hole and the transmission line. As a result, the conventional switching circuits have disadvantages that there is a limit on the isolation that may be ensured per a unit cell, and the insertion loss is also increased with the increase in the impedance of the transmission line since only two via holes are disposed in each of the switching circuits. Also, the conventional switching circuits have a problem that the manufacturing cost of chips is also still high like the convention multi-stage shunt technology.